Form_Load() 'event' or Override OnLoad()

Question

I would like someone to try and explain the difference between these. More specifically, example usage scenario's.

I am refactoring some Windows Form code and a Form has some code in the Form_Load() event and also in a protected override void OnLoad() event that calls base.OnLoad(e); Now I traced it and the Override fires first and then immediately fires off the event version.

So, which one is typically used and why?

Thanks,

Answer 1

OnLoad raises the Load event, which Form_Load handles. See the MSDN article.

So, in your code, at the end of your initialization function, you could call the OnLoad function. The base class will then call your event handlers (Form_Load in this case), which you may have set in your initialization code.

Personally, I would place my code in the Form_Init and leave OnLoad for initiating the event, since I'm handling the event, although you can defend either with good reason, as you see from your answers.

Answer 2

The base.OnLoad method invokes the Load-event of the Form.

protected override void OnLoad(EventArgs e)
{
  // do stuff before Load-event is raised
  base.OnLoad(e);
  // do stuff after Load-event is raised
}

If you don't specifically need to perform stuff before the Load-event is raised, placing the code in OnLoad after base.OnLoad(e) is as good as placing it in the Form_Load event handler.

Answer 3

You should always override OnLoad(). Using the event is only appropriate when another class would be interested in the event. Which is what events are for. Another class being interested in the Load event is very rare, only really useful to do window arrangement stuff.

Still, the Load event works well with the designer and VB6 programmers are very comfortable with it. It isn't horribly wrong, you'd only get in trouble when you start inheriting the form and code doesn't run in the right order.

Most code that now gets put in the Load event really belongs in the constructor. You only need OnLoad if:

• You need to know the exact size and position of the window. OnLoad is best, the window Handle is created and the user preferences are applied (title and border size) and the form was rescaled as directed by the Form.AutoScaleMode property. The window is not yet visible, a very good time to move the window somewhere else or to arrange the child controls.
• You have code that needs the Handle property. This is subtle, you cannot always tell. Having code like that in the constructor is unhealthy, the window gets created before the constructor completed. It usually comes to a good end but it can make creating the form very slow. Easy to diagnose from the Call Stack window.
• To avoid a bug in the MDI implementation. If you create an MDI child in the parent constructor then you'll get duplicated glyphs, visible when you maximize the child. Create the child in OnLoad instead.